1. Field of the Invention
The present invention relates generally to control systems used for data storage devices, and more particularly, to a system and method providing a command aging method that generates an incentive term for use with expected access times when selecting commands for execution in data storage devices.
2. Description of Related Art.
It is well known in the art to store data on magnetic or optical disk drives. Generally, data is stored on a disk drive on one or more tracks of predetermined format disposed on a disk-shaped recording media. The data is written to and read from the tracks in response to a read/write command issued by a host computer.
When a read/write command is issued by a host computer, it may be queued with other commands for the disk drive. Throughput in a queued disk drive can be optimized by selecting the command in the queue that has the minimum expected access time (EAT). EAT is the time required from the completion of one command to the start of the next, plus one revolution multiplied by the probability of a seek, read, or write error.
For example, consider a system with a single priority level, or where only the commands that are within one priority are selected. Consequently, at each selection time, commands are selected solely upon their EAT and not any other criterion.
The average service time (the time from a command's entrance into the queue until its exit from the queue), can be determined from the throughput and the average queue size. The average service time will be minimized by maximizing the throughput; however, some commands will remain in the queue far longer than average. Consequently, there is a need for a mechanism to place an upper limit on the maximum amount of time a command is queued, when excessive service times cannot be tolerated.
Typically, a system will force a command to be selected when it reaches some age limit, independent of its EAT. This may replace a potentially very fast command with a very slow one, thereby affecting throughput and average service time. The present invention, on the other hand, allows the system to control the incentive for aging commands to leave the queue, thus controlling the effect on throughput.